The well-known cathode ray tube (CRT) is widely used for television (TV) and computer displays. Other display technologies such as the transmissive liquid crystal display (LCD) panel are widely used in certain specialized applications such as displays for portable computers and video projectors.
Market demand is continuously increasing for video displays with higher resolution, greater brightness, lower power, lighter weight, and more compact size. But, as these requirements become more and more stringent, the limitations of conventional CRTs and LCDs become apparent. Microdisplays the size of a silicon chip offer advantages over conventional technologies in resolution, brightness, power, and size. Such microdisplays are often referred to as spatial light modulators (SLMs) since, in many applications, (for example, video projection) they do not generate light directly but instead produce an image by modulating an incident light source. In projection applications, an SLM modulates a bright light source such as an arc lamp, forming a projected image on a screen. In other applications such as ultraportable or head-mounted displays, an image on the surface of the SLM may in fact be viewed by the user directly or through magnification optics.
CRTs currently dominate the market for desktop monitors and consumer TVs. But large CRTs are very bulky and expensive. LCD panels are much lighter and thinner than CRTs, but are prohibitively expensive to manufacture in sizes competitive with large CRTs. SLM microdisplays enable cost-effective and compact projection displays, reducing the bulk and cost of large computer monitors and TVs.
Transmissive LCD microdisplays are currently the technology of choice for video projection systems. But, one disadvantage of LCDs is that they require a source of polarized light. LCDs are therefore optically inefficient. Without expensive polarization conversion optics, LCDs are limited to less than 50%-efficient use of an unpolarized light source. Unlike LCDs, micromirror-based SLM displays can use unpolarized light. Using unpolarized light allows projection displays using micromirror SLMs to achieve greater brightness than LCD-based projectors with the same light source, or equivalent brightness with a smaller, lower-power, cheaper light source.
The general operation and architecture of SLMs and SLM-based displays is well known in the industry as shown, for example, in U.S. Pat. No. 6,046,840, U.S. Pat. No. 5,835,256, U.S. Pat. No. 5,311,360, U.S. Pat. No. 4,566,935, and U.S. Pat. No. 4,367,924, the disclosures of which are each incorporated herein by reference.
FIG. 1 shows the optical design of a typical micromirror SLM-based projection display system. A light source 20 and associated optical system, comprising optical elements 2a, 2b, and 2c, focus a light beam 6 onto the SLM 4. The pixels of the SLM are individually controllable and an image is formed by modulating the incident light beam 6 as desired at each pixel. Micromirror-based projection displays typically modulate the direction of the incident light. For example, to produce a bright pixel in the projected image, the state of the SLM pixel may be set such that the light from that pixel is directed into the projection lens 8. To produce a dark pixel in the projected image, the state of the SLM pixel is set such that the light is directed away from the projection lens 8. Other technologies, such as reflective and transmissive LCDs, use other modulation techniques such as techniques in which the polarization or intensity of the light is modulated.
Modulated light from each SLM pixel passes through a projection lens 8 and is projected on a viewing screen 10, which shows an image composed of bright and dark pixels corresponding to the image data loaded into the SLM 4.
A “field-sequential color” (FSC) color display may be generated by temporally interleaving separate images in different colors, typically the additive primaries red, green, and blue, though often a white image is added to boost the brightness of the image. This may be accomplished as described in the prior art using a color filter wheel 12 as shown in FIG. 1. As color wheel 12 rotates rapidly, the color of the projected image cycles rapidly between the desired colors. The image on the SLM is synchronized to the wheel such that the different color fields of the full-color image are displayed in sequence. When the color of the light source is varied rapidly enough, the human eye perceives the sequential color fields as a single full-color image. Grayscale in the image can be achieved by pulse width modulation, such as set forth in WO 01/84531 to Richards, incorporated herein by reference.
Other illumination methods may be used to produce a field-sequential color display. For example, in an ultraportable display, colored LEDs could be used for the light source. Instead of using a color wheel, the LEDs may simply be switched on and off as desired.
An additional color technique is to use more than one SLM, typically one per color, and combine their images optically. This solution is bulkier and more expensive than a single-SLM solution, but allows the highest brightness levels for digital cinema and high-end video projection.
In a CRT or conventional LCD panel the brightness of any pixel is an analog value, continuously variable between light and dark. In fast SLMs, for example those based on micromirrors, other MEMS structures, or some types of LCDs, one can operate the pixels in a digital manner. That is, pixels of these devices are driven to one of two states: flly on (bright) or fully off (dark).
To produce the perception of a grayscale or full-color image using such a digital SLM, it is necessary to rapidly modulate the pixels of the display between on and off states such that the average of each pixel's modulated brightness waveform corresponds to the desired ‘analog’ brightness for that pixel. This technique is generally referred to as pulse-width modulation (PWM). Above a certain modulation frequency, the human eye and brain integrate a pixel's rapidly varying brightness (and color, in a field-sequential color display) and perceive an effective ‘analog’ brightness (and color) determined by the pixel's average illumination over a video frame.
It is generally advantageous to drive the pixels of a digital SLM with as large a voltage as possible. For example, in a MEMS based microdisplay such as those disclosed in U.S. Pat. Nos. 5,835,256 and 6,046,840 (both to Huibers and incorporated herein by reference) a large actuation voltage increases the available electrostatic force available to move the micromechanical pixel elements. Greater electrostatic forces provide more operating margin for the micromechanical elements—increasing yield—and actuate them more reliably and robustly over variations in processing and environment. Greater electrostatic forces also allow the hinges of the MEMS structures to be made correspondingly stiffer; stiffer hinges may be advantageous since the material films used to fabricate them may be made thicker and therefore less sensitive to process variability, improving yield. A further benefit of larger electrostatic forces and stiffer micromechanical hinges is that more force is available to overcome the stiction force that is present between contacting MEMS structures. The pixel switching speed may also be improved by raising the drive voltage to the pixel, allowing higher frame rates, or greater color bit depth to be achieved.
Higher actuation voltages may be of benefit for other SLM types, as well. For example, many LCD materials can be made to switch states faster with a larger drive voltage, allowing greater frame rates and/or color bit depth.
Designing high-voltage circuitry to individually control the pixels of an SLM is challenging. Due to fundamental principles of semiconductor physics, transistors that tolerate higher voltages must be physically larger than those that tolerate lower voltage. Larger, HV-tolerant transistors may unacceptably reduce the cell area available for other critical cell components (such as the capacitor in a DRAM-type cell) or not fit in the cell at all. On the other hand, it is desirable for cost reasons to minimize the physical size of the microdisplay pixel. Thus it is highly desirable to improve the maximum drive voltage of an SLM pixel circuit while maintaining small pixel size.
The 1-transistor, 1-capacitor (1T1C) DRAM charge-storage cell illustrated in FIG. 2 is a well-known circuit for storing a data voltage in many applications. The 1T1C circuit is widely applied to ordinary data storage in commercial DRAMs and other integrated-circuit applications. This 1T1C circuit structure is also used in display applications—TFT active-matrix LCD panels work on the same principle, and the idea has also been applied to microdisplays as described in U.S. Pat. No. 5,671,083.
A well-known drawback of the 1T1C DRAM circuit is the nonideal performance of the pass transistor. Ideally the transistor would act as a perfect switch; the voltage on the bitline 101 would be passed through without change to the storage node 102 when the wordline 100 is activated. A real NMOS pass transistor can pull the stored cell voltage 102 down all the way to the minimum bitline 101 voltage Vbmin. However the highest voltage that can be driven on the cell storage node 102 is limited by the threshold voltage of the pass transistor 104. The transistor can only pull the voltage Vc on cell storage node 102 up to no higher than Vg-Vt, where Vg is the wordline ‘on’ voltage applied to the gate of the pass transistor, and Vt is the transistor's threshold voltage. To maximize Vc, Vg would typically be driven to Vdd, the maximum supply voltage allowed by the breakdown limits of the IC process in which the circuit is fabricated. In this case the resulting cell voltage will be Vdd-Vt, whereas the bitline voltage is driven to Vdd. So the transistors of the cell are themselves capable of tolerating Vc=Vdd, but the maximum actual Vc is limited to only Vdd-Vt due to the inherent properties of the 1T1C cell circuit.
Compounding the problem is the well-known MOSFET ‘body effect.’ This causes the effective Vt of the pass transistor to become even larger as the transistor's source node (the storage node 102) rises in voltage, as is the case when the cell is charged to Vdd-Vt. This reduces the maximum available output Vc still further.
One of ordinary skill in the art of circuit design will appreciate that the above discussion applies equally well to a PMOS transistor, which will suffer similar voltage degradation when pulling the stored voltage down.
Clearly, a circuit that could store the full supply voltage of Vdd on Vc would be more desirable. Many candidate circuit designs are impractical in a microdisplay due to the small pixel cell size. For example, a 2-transistor (PMOS+NMOS) pass gate could be used to get the full supply voltage swing on the storage node. However, the large design rules required for adequate spacing between PMOS and NMOS devices and their associated wells are impractical for small pixel sizes. Alternatively, an SRAM pixel cell could be used, but this cell requires 6 transistors that also would likely not fit in the available pixel area. Alternatively a PMOS transistor or diode could be added to the cell to ‘precharge’ the pixel cell high before writing it to a low value, but high-voltage PMOS devices or diodes also occupy an unacceptably large amount of area for small microdisplay pixels due to their associated well implants. A further alternative would be to increase the gate voltage to the wordline by Vt over the maximum cell voltage. However, this would require even higher-voltage transistors than the high-voltage transistor in the cell. Fabricating two types of high voltage (HV) transistors would increase cost and the cell would still not be able to use the highest-voltage transistors available.